X-ray tube apparatus of a rotating anode type

Abstract: In an X-ray tube apparatus of a rotating anode type, a stator (23) surrounds an anode rotary structure (15) and an insulating container section (17) placed around the outer periphery of a stationary structure (21) such that a portion of its coil conductor (31) located near the anode target (19) side constitutes an expanding flared coil conductor portion (31a). Therefore, it is possible, for the X-ray tube equipped with an envelope having a large-diameter metal section and small-diameter insulating container section, to shorten the axial length from an anode target of the X-ray tube to a far end of the rotary structure and to suppress the build-up of electric charges on the inner surface of the insulating container section.

Description: The present invention relates to an X-ray tube apparatus of a rotating anode type according to the preamble of claim 1 and, in particular, an improvement in the structure of a rotating anode type X-ray tube as a vacuum container equipped with a metal container section for receiving an anode target, in the structure of an X-ray tube holding housing for holding the rotating anode type X-ray tube and in the structure of a stator for rotational drive. As well-known in the prior art, the rotating anode type X-ray tube is mounted within an X-ray tube holding housing filled with an insulating oil. The X-ray tube apparatus of a rotating anode type is equipped with a stator of an electromagnetic induction motor for rotating a rotary structure of the X-ray tube at high speeds. The stator above is comprised of an iron core/coil conductor-combined unit and located near the outer periphery of a vacuum envelope for housing the rotary structure in the X-ray tube corresponding to a rotor of the motor. As shown in FIG. 1, the stator 13 is constructed by a stator coil conductor 12 wound along a number of slits formed in a cylindrical iron core 11, that is, a core comprised of stacked thin sheet rings made of a ferromagnetic material. On the other hand, the X-ray tube 14 is equipped, as well known in the art, with a glass container section 17 of a vacuum envelope 16 surrounding a rotary structure 15. A disc-like anode target 19 is arranged in the vacuum envelope 16 at a metal container section 18 of a large diameter. The anode target 19 is fixed by a rotation shaft 20 to the rotary structure 15 and supported there. The rotary structure 15 is rotatably held on a stationary structure 21 by bearing means not shown. In FIG. 1, reference numeral 18a denotes a corona ring extending from the metal container section; 17a, an expanding flared section of the glass container section; and 17b, a small-diameter cylindrical section of the glass container section. The stator 13 is arranged near the outer periphery of the small-diameter cylindrical section 17b of the glass container section. A rotation magnetic field is generated mainly on the inside of the iron core 11, acting upon the rotary structure 15 and hence rotating the rotary structure at high speeds. With the conventional X-ray tube apparatus having a structure as shown in FIG. 1, the coil conductor 12 of the stator 13 linearly extends toward the anode target side and the ion core 11 is relatively spaced far apart from the anode target 19. From the structural and operational condition of the X-ray tube apparatus, usually, the metal container section 18 of the vacuum container (envelope) is held at a ground potential and applied a high positive voltage of, for example, 75 kV is to the anode target 19. For this reason, an interval G between the anode target 19 and the metal container section 18 of the vacuum container is maintained at a distance enough great to withstand such a high voltage difference during operation. The axial distance H from the lower end of the anode target 19 to that of the rotary structure 15 is increased to an undesired extent. Further, the iron core 11 of the stator 13, together with the X-ray tube holding housing, is connected to a ground potential and the iron core and the coil conductor are substantially connected to the ground D.C. potential, even if an AC drive voltage is applied to a coil conductor 12 at the operation of the X-ray tube apparatus. During the operation of the X-ray tube apparatus, a great potential gradient is involved on the inner surface of the expanding flared section 17a of the glass container section due to a potential distribution created between the inside corner portion of the upper end of the stator 13 and the rotary structure in the X-ray tube. Floating electrons e entering into the space between the corona ring 18a and the rotary structure 15 straightly reach the inner surface of the expanding flared section 17a which is charged up by the floating electrodes. This is liable to develop an undesired discharge. Such a known X-ray tube apparatus according to the preamble of claim 1 is described in WO-A-9308587. It includes means for preventing the build-up of charge on the anode glass portion of the tube envelope where the glass flares by constraining the equipotential lines of the electric field in the vicinity of the flare parallel to the flare surface. Parallelism may be achieved by controlling the angle of flare and sealing the flare directly to the metal section, modifying the anode rotor to include a flare conforming to the glass flare, and including a ground plane screen in the tube housing. Another X-ray tube apparatus is known from DE-A-3341976 in which the stator substantially extends along the flared portion of one of the vessels constituing the X-ray tube. Thereby, two different stator coils are stacked and shifted from one another in order to be adapted to the shape of the vessel. Since the stator coils are parallel to the axis of the vessel, the magnetic field generated by these coils is substantially perpendicular to the axis of the vessel. Due to this arrangement of the coils the magnetic field at the flared portion of the vessel is very weak. Finally, EP-A-552808 teaches a rotating anode X-ray tube in which leakage of a liquid metal lubricant can be prevented, and a stable bearing operation can be maintained. The liquid metal lubricant is supplied to bearing portions between a rotary member and a stationary member. It is accordingly an object of the present invention to provide an X-ray tube apparatus of a rotating anode type which can shorten an axial distance from the lower end of an anode target to the lower end of a rotary structure to provide a compact unit and can supress the build-up of electric charges on the inner surface of an expanding flared section of an insulating container section so as to prevent occurrence of a discharge there. According to the present invention an X-ray tube apparatus of a rotary anode type is provided as defined in claim 1. Further favourable embodiments are set out in the subclaims. With such an X-ray tube apparatus of the rotating anode type, an axial distance of the tube from the lower end of the anode target to the lower end of its rotary structure can be shortened to provide a compact unit and it is possible to suppress electric charges from being accumulated on the inner surface of the expanding flared section of the insulating container section resulting from an action of an electromagnetic field by the expanding section of the stator's coil structure and to thereby ensure a stable operation, while achieving less discharge. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: FIG. 1 is a cross-sectional view, partly taken away, diagrammatically showing part of a structure of a conventional X-ray tube apparatus; FIG. 2 is a cross-sectional view, partly taken away, diagrammatically showing a major section of an X-ray tube apparatus of a rotating anode type according to an embodiment of the present invention; FIG. 3 is an expanded, cross-sectional view, partly taken away, showing a major section of the apparatus of FIG. 2; FIG. 4A is a side view showing a stationary structure in FIG. 2, FIG. 4B is a cross-sectional view, partly taken away, showing a thrust ring in FIG. 2, FIG. 4C is a top view showing a bearing as viewed along line C-C in FIG. 4, and FIG. 4D is a top view showing a bearing as viewed along line D-D in FIG. 4; and FIG. 5 is an expanded cross-sectional view, partly taken away, for explaining the effects of the embodiment of FIG. 2. An X-ray tube apparatus according to one embodiment of the present invention will be explained below with reference to FIGS. 2 to 5. Throughout the drawings, the same reference numerals are employed to designate the same parts or elements. The X-ray tube apparatus according to the embodiment of the present invention has the following structure. That is, a holding housing 22 for holding an X-ray tube 14 of a rotating anode type is filled with an insulating oil and the end portion of a stationary structure 21 of the X-ray tube is fixedly threaded to an insulating support frame 29 within the X-ray tube holding housing 22, the support frame 29 being made of, for example, plastics. Within the holding housing 22 a stator 23 is fixedly held on a support angle 24 and insulating support frame 29. Further, the holding housing 22 has a shielding lead layer 25 lined with a lead and a connection terminal 26 connected to a high-tension cable. In the X-ray tube 14, a disc-like anode target 19 made of a heavy metal is arranged in a metal container section or a large-diameter section 18 of a vacuum container or envelope 16 and the anode target 19 is fixed to a rotation shaft 20 which is in turn fixed by the rotation shaft 20 to a cylindrical rotary structure 15. The rotary structure 15 is rotatably fitted into the stationary structure 21 through bearing means as will be set out below and supported there. The end portion of the metal container section 18 of the vacuum container 16 extends substantially along the curved surface of an outer periphery of the target 19 and has its diameter reduced gradually and a corona ring 18a is provided at the lower end. The rotary structure 15 is received in an insulating container section 17 made of glass. As shown in FIGS. 2 and 3, the insulating container section 17 has an outwardly expanding flared section 17a on the target side and an upper end section extending along the outer periphery of the corona ring 18a and joined to the lower end of the metal container section 18 by a sealing metal ring 28. The insulating container section 17 has a small-diameter cylindrical section 17b straightly extending in a close proximity relation to the outer periphery of the rotary structure 15. The small-diameter cylindrical section 17b has its lower end welded, in a hermetically sealing way, to the outer peripheral portion of the anode stationary structure 21 by a sealing metal ring 27a and auxiliary metal ring 27b. As shown in FIG. 3, the cylindrical rotary structure 15 has a ferromagnetic cylindrical section 15a made of iron or hard iron alloy and a cylindrical section 15b fixed to the outer periphery of the cylindrical section 15a and made of a good conduction such as copper or copper alloy. A shoulder 15c, on the shaft-side, of the cylindrical section is positioned in an inside space of a central recess 19a in a rear surface side of the anode target 19. Further, a thrust ring 15e made of iron or iron alloy is fixed to an open end section 15d of the rotary structure 15 by a plurality of screws. Two sets of dynamic pressure bearings, radial slide bearings 41, 42 and thrust slide bearings 43, 44, are provided at those fitting portions between the rotary structure 15 and the stationary structure 21. The two radial slide bearings 41, 42 are provided in a spaced-apart relation to the axial direction of the rotation shaft and have two sets of herringbone pattern spiral grooves 41a, 42a provided in the outer peripheral surface of the stationary structure 21 as shown in FIG. 4A. The spiral groove 41a is located near the anode target and has a length about double that of the other spiral groove 42a along the axial direction of the rotation shaft and hence has a relatively great bearing-withstand load capability. A small-diameter section 21b of the stationary structure 21 is provided at an intermediate area between the spiral grooves 41a and 42a. The stationary structure 21 is made of a hard iron alloy. The thrust slide bearing 43 has circular herringbone pattern-like spiral grooves on the end surface 21a of the anode stationary structure as shown in FIG. 4C while, on the other hand, the thrust slide bearing 44 has a circular herringbone pattern-like spiral grooves 44a provided on the upper surface of the thrust ring 15 placed in contact with a step surface of the lower portion of the stationary structure. The slide bearing surfaces contacting with the associated spiral-grooved bearings may be provided as simply flat surfaces or spiral-grooved surfaces as required. It is to be noted that the bearing surfaces of the rotary structure and stationary structure are such that a gap of about 20 µm is maintained relative to these bearings during the rotation operation of the apparatus. The stationary structure 21 has a lubricant holding chamber 45 bored in a direction of its center axis as shown in FIG. 4C and a lubricant passage 46 pierced through the small-diameter section 21b in a crisscross relation as shown in FIG. 4A. A liquid metal lubricant, not shown, such as a gallium/indium/tin-based alloy is applied into the respective spiral grooves, bearing gaps, lubricant holding chamber and lubricant passage, noting that it becomes a liquid state at least during operation. As shown in FIGS. 3 and 5, the stator 23 has a coil conductor 31 arranged along a number of axial slits provided on the inside of a circular iron core 30 and turned at the upper and lower sides. A coil conductor section, in particular, on the metal container side has an expanding flared coil conductor section 31a. In the case of this embodiment, the coil conductor expanding section 31a is externally flared along the expanding flared section 17a of the insulating container section. The axial length La of the flared coil conductor section 31a is determined to be over 20% of the whole axial length Lb of the stator 23. The practical upper limit is set to be desirably about 60%. Further, the flared coil conductor section 31a may be of such a type that it is expanded in a lateral direction substantially at right-angle relation or it has its inner coil surface only expanded in a flared way. An insulating cylindrical member 32 made of plastics is interposed between the stator 23 and the insulating container section 17 so as to enhance electrical insulation. The anode target-side portion of the insulating cylindrical member 32 is expanded, as an expanding flared portion, along the expanding flared section 17a of the insulating container section and extends further outwardly than the forward end of the expanding flared coil conductor section 31a. The stator has its iron core 30 provided preferably at an intermediate area between the two radial slide bearings 41 and 42, that is, in a position substantially corresponding to the small-diameter section 21b of the stationary structure. By doing so, a rotation magnetic field created by the stator is not exerted on the major portion of the spiral grooves of the respective dynamic pressure type slide bearing, thus alleviating undesirable causes, such as the generation of unwanted heat or the promotion of a chemical reaction produced in the liquid metal lubricant. This proves effective to maintain a stable bearing operation. In this way, the anode target-side coil conductor of the stator is laterally expanded along the expanding flared section 17a of the insulating container section and in a relatively close proximity relation to the latter, so that the stator can be located near the anode target side. As a result, the axial distance (corresponding to a dimension H in FIG. 1) from the lower end, that is, the rear end side, of the anode target to the lower end of the rotary structure can be shortened to provide a compact unit. Further, the expanding flared coil conductor section 31a constitutes a conductor of a substantial ground potential, thus leading to the alleviation of a potential gradient at its neighboring insulating container section, in particular, at the inner surface of the expanding flared section, and hence to the suppression of the charging of floating electrons. Further, a rotation magnetic field created from the expanding flared coil conductor section of the stator is much weaker than that generated from the iron core, but, as indicated by reference symbol F in FIG. 5, it is bulged toward the anode target side, passes through the rotary structure and stationary structure and reaches a reverse side. Even if, therefore, floating electrons e enter into space between the corona ring of the metal container section and the anode rotary structure, they reach the outer peripheral surface of the rotary structure (anode potential), while being rotated around the magnetic flux as indicated by a dotted line in FIG. 5, due to both the leakage fields F and electric field distribution in that space, so that they are caught there. Even from this it is also possible to suppress the charging of electrons on the insulating container section, in particular, on its expanding flared inner surface and hence to suppress any discharge resulting therefrom. It is to be noted that the bearing may be comprised of not only the above-mentioned dynamic pressure type bearing but also a ball bearing or their combination. As explained above, according to the X-ray tube apparatus it is possible to shorten the axial distance from the lower end of the anode target to the lower end of the rotary structure and hence to provide a compact apparatus. It is also possible to suppress the charging of electrons on the inner surface of the insulating container section and hence to achieve the suppression of a resultant discharge and to obtain a stable operation.

Claims: An X-ray tube apparatus of a rotating anode type, comprising: (1) a rotary anode type X-ray tube (14) including (a) disc-like anode target (19), (b) a rotary structure (15) to which the anode target (19) is fixed, (c) a stationary structure (21) for supporting the rotary structure (15), (d) bearing means (41, 42) provided between the rotary structure (15) and the stationary structure (21), for rotatably bearing the rotary structure (15) around the stationary structure (21) and (e) an envelope (16) having a large-diameter metal container section (18) and a small-diameter insulating container section (17); (2) an X-ray tube holding housing (22) for holding the X-ray tube (14) therein; and (3) a cylindical stator (23) comprised of an iron core (30) and coil conductor (31) wound around the iron core (30), in the direction of the longitudinal axis of the X-ray tube, the iron core (30) and coil conductor (31) surrounding the rotary structure (15) of the X-ray tube and insulating container section (17) of the envelope (16) within the X-ray tube housing (22), said insulating container section (17) has an expanding flared end portion and hermetically joined to the metal container section (18), the disc-like anode target (19) is arranged within the metal container section (18), and the rotary structure (15) and stationary structure (21) are received in the insulating container section (17); characterized in that said cylindrical stator (23) has a flared coil conductor portion (31a) located near the metal container section (18) and expanded substantially along the expanding flared end section of the insulating container section (17), the direction of the coil conductor winding following that of said flared portion of said insulating container and an axial length defined by the expanding flared section of the coil conductor (31) is so set as to be over 20% of a whole axial length of the stator (23). Apparatus according to claim 1, characterized in that the bearing means (41, 42) is comprised of dynamic pressure type slide bearings having spiral grooves applied with a liquid metal lubricant. Apparatus according to claim 1, characterized in that the bearing means (41, 42) is comprised of two dynamic pressure type slide bearings (41, 42) spaced apart in an axial direction of the X-ray tube (14) and having spiral grooves applied with a liquid metal lubricant and the core (30) of the stator (23) is located in a position corresponding to an area between the two slide bearings (41, 42). Apparatus according to claim 1, characterized in that the anode target (19) has a recess and the rotary structure (15) has its shoulder portion located in the recess of the anode target (19).

Inventors: KITAMI TAKAYUKI, ONO KATSUHIRO, KITAMI, TAKAYUKI, ONO, KATSUHIRO, KITAMI, TAKAYUKI, C/O INTELLECTUAL PROPERTY DIV., ONO, KATSUHIRO, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1994
Main classification: H01J35
IPC codes: H01J35, H05G1